// ドキュメントを保存します

 SaveFileDialog dlg = this.GetFileDialog();

{

            if (dlg != null)

            {

                  using (Stream stream = dlg.OpenFile())

                  {

                         pdf.Save(stream);

                  }

            }

      }

      private SaveFileDialog GetFileDialog()

      {

            SaveFileDialog dlg = new SaveFileDialog();

            dlg.DefaultExt = ".pdf";

            bool? dr = dlg.ShowDialog();

            return ((dr.HasValue && dr.Value) ? dlg : null);

      }